Influence of composition and annealing conditions on the site-occupation in the σ-phase of Fe–Cr and Fe–V systems

Neutron diffraction technique was used to study the site-occupation in the σ-phase in the Fe–Cr and Fe–V systems. It was found that all five sites A, B, C, D and E are “mixed”, i.e. occupied by both elements. The occupation is neither random – the degree of randomness increases with Fe content – nor regular, i.e. sites A and D are predominantly occupied by Fe atoms while B, C and E sites are occupied preferentially by Cr or V atoms. For all five sites the increase of Cr(V) concentration results in an irregular decrease of the number of Fe atoms on each site, the rate of decrease being the smallest for the sites A and D. In the Fe–Cr system the population of A with Fe atoms, NA, is similar to that of D, ND, while for the system Fe–V ND > NA and the difference increases with V content. The lowest Fe population in both investigated systems has B, but NB(V) < NB(Cr). NC = NE for Fe–V, while NC > NE for Fe–Cr. The influence of sample preparation conditions (plastic deformation prior to phase transformation, annealing time, ta, and temperature of annealing, Ta) were also tested on a σ-Fe53.8Cr46.2 sample. In general the influence of these conditions on the site population is small. The largest effect had the plastic deformation, and the smallest one the annealing time in case of non-deformed samples. The most insensitive sites were revealed to be D and E, and the most sensitive ones were B and A.